FROM TWELVE TO THIRTY-SIX SOMITES 163 
Metenc Isth. crf]. Mesenc. 
Fic. 93. —Entire embryo of 27s viewed as a 
transparent object from above. 
a.a.1,a.a.2,a.a.3, First, second, and third 
aortic arches. Car., Carotid loop. Ret., Retina. 
v. C. 1, v. C. 2, First and second visceral clefts. 
Other abbreviations as before. x 20. 
